Inverse Synthetic Aperture Radar (ISAR) processing is a technique for generating a three-dimensional (range, doppler, and intensity) image of a complex radar target. This kind of image allows a radar operator to classify targets at ranges beyond those available using visual methods. ISAR system operational effectiveness in classifying ships is clearly observable in the An/APS-137(V) radar systems on the P-3, CS-3B, and C-130 surveillance aircraft. Additionally, surface platforms such as cruisers and fast patrol boats are attractive platforms for these types of radar systems. Recently, ISAR systems have proven effective for classifying aircraft in a variety of applications.
The Navy/Marine surveillance community recognizes the value of ISAR systems. At the same time, the community recognizes that ISAR processing methods and systems employing this methods fall short of their full potential use. To address this problem, some designers seek to improve range resolution of ISAR systems. For example, Advanced Profile, a radar test bed system having ISAR capability, shows the value of improved resolution. The Advanced Profile test bed system classifies smaller targets and permits operators to examine finer detail on larger aircraft. This yields improved classification results and enhanced damage assessment capabilities.
For known systems, component capabilities limit resolution improvement. For example, fielded ISAR systems use surface acoustic wave (SAW) filters to achieve high range resolution. These analog devices become more difficult to produce as the time-bandwidth product increases. Following impulse compression and down-conversion to baseband, these systems digitize a compressed waveform and digitally process doppler (cross-range) dimensions. In these systems, however, throughput capability of the analog-to-digital conversion, buffering, and digital processing limit resolution.
Known ISAR systems also have a "dynamic" limitation on resolution improvement. Target movement of a few tenths of a wavelength during an aperture time causes phase modulation of the return signal. Special processing of this phase modulation results in the cross-range dimension of an image. The cross-range resolution relates to the aperture time. However, larger-scale motion of the target during aperture formation causes amplitude modulation of the return signal.
Amplitude modulation of the return signal causes cross-range dimensional spreading of the return signal spectrum. Spreading of the return signal spectrum is known as doppler smearing. Doppler smearing causes the cross-range resolution to degrade. Thus, known systems reach a point where increasing a pulse bandwidth actually degrades the cross-range resolution. This is contrary to the expected improved range resolution which should occur with increasing bandwidth. In fact, no improvement in range resolution can occur, because of target drifts in range. For example, if a target drifts in range a distance that is significant relative to the range resolution for a particular bandwidth, it is likely that degraded range resolution will occur. The doppler smearing that occurs in the ISAR image is said to be the result of "range migration."
Consequently, there is a need for a method and system to overcome the doppler and range resolution problems that range migration causes.
There is a need for an ISAR processing method and system that improves doppler and range resolutions but is not limited by the input capability of analog-to-digital conversion, buffering, and digital processing components of known ISAR systems.
Further, there is a need for a method and system that overcomes doppler frequency smearing effects that exist in known ISAR systems.